A system where multiple computational units are interconnected is frequently used to provide increased processing power. The method by which the computational units share information is of critical importance to the performance of the system. While many connections may be made electronically, it is advantageous to use optical connections for advantages in speed, length of connection, power dissipation, noise immunity, lower electromagnetic emissions, and physical size. There is a class of signals, namely AC-coupled or AC components, which can be easily transposed from an electrical to an optical connection. However, another class of signals, DC coupled signals or DC coupled components do not easily transpose from an electrical connection to an optical one. The primary reason for this is that the high speed (Gb/s-class and greater) optical links which are used for these tapes of connections are typically constructed as AC coupled and not DC coupled. The other reason for this is the uni-directional nature of optical communication. The DC coupled components are commonly referred to as auxiliary signals, or control signals, or power management/reset-initialization signals. Many of these DC coupled signals are unidirectional, i.e. one end of the link sets the logic level and the other end receives it. Unidirectional DC coupled signal can be easily transmitted over AC coupled links, both electrically and optically, using a variety of techniques that are well known to those skilled in the art. However, there is a class of DC coupled signals which are bidirectional, i.e. either end of a link can set the logic level on a common wire and all ends receive it. A wired-and bus with two or more open drain circuits connecting to the same wire and a single shared pull-up resistor or load is one typical example of a bi-directional DC coupled signal. These bidirectional DC coupled signals are not trivial to transmit over an AC coupled optical links due to the unidirectional nature of optical links. There is a third class of DC coupled signal which do not originate from either end of a communication link but, rather, are injected by an independent system or management controller either locally and/or remotely.
There are three main approaches to transmitting the unidirectional auxiliary signals. These three main approaches can be characterized as the three wire approach, the two wire approach and the one wire approach. FIG. I is an example of the three wire approach. Using three wires, the clock (CLK), Data and Data_valid signals are transmitted over the optical link. The CLK is used to sample inputs and generate the outputs and the Data_valid denotes the beginning of valid information. The next approach is the two wire approach. In this approach, there are only the CLK and Data transmission lines of the three wire approach. A DC balanced serializer IC such as the MAX9209 produced by Maxim Inc. is one example of the two wire approach for unidirectional signals. The final method, not illustrated, is the one wire approach. This method uses a clock and data recovery process to determine the data. A DC balanced serializer IC such as the MAX9247 produced by Maxim Inc. is one example of the one wire approach for unidirectional signals. The one wire approach using a single optical wire for each DC coupled signal that is transmitted is not cost effective nor space and power efficient and is undesirable for high speed optical transmissions. Bidirectional signals must be treated differently. Because of the unidirectional nature of fiber optic links, a minimum of two optical lanes are needed (one in each direction) and some additional logic at each end to combine the state of each unidirectional lane back into the original bidirectional signal.